1. Field of the Invention
The present invention relates to an optical transmitter for use with an optical communication system and, more particularly, to improvements of an optical modulator having an optical modulator.
Heretofore, optical transmitters used by optical communication systems have adopted the so-called direct modulation as their operating principle. The method involves modulating the current that flows through laser diodes by use of data signals. A major drawback of direct modulation is the difficulty in implementing long-distance data transmission due to wavelength dispersion. Efforts of recent years to circumvent the disadvantage have led to the development of external modulation. This is the kind of optical modulation that is highly immune to the adverse effects of wavelength dispersion over optical fiber cables. For example, what is known as the LiNbO.sub.3 Mach-Zehnder modulator has drawn attention For its excellent modulation characteristic and high resistance to wavelength dispersion. Optical transmitters based on the external modulation principle are subject to a number of requirements: (a) the operating point of the optical modulator should be controlled to be stable; (b) the optical modulator should be driven with a low voltage; (c) there should be minimum waveform distortion attributable to the capacitor between a driving circuit and the electrodes of the optical modulator as well as to the capacitor between the electrodes and a terminating resistor; and (d) there should be minimum changes in modulation characteristic which are attributable to abrupt changes in the so-called mark rate. It is also required that the presence or absence of chirping (dynamic wavelength fluctuation) in the optical signal output by the optical modulator be optional. For example, where long-distance transmission is effected using a wavelength that approximately matches the zero-dispersion wavelength of the optical fiber and where the dispersion value can be either positive or negative, there should be no chirping. On the or, her hand, where the polarity of the wavelength dispersion off the optical fiber is predetermined and where optical pulse compression may be effected using a kind of chirping that corresponds to the dispersion polarity, the presence of chirping should be selected.
2. Description of the Related Art
The typical Mach-Zehnder modulator comprises an input port that receives light from a light source, a pair of branching waveguides that transmit the light received through the input port after branching the light in two directions, an output port that converges the branched light streams coming out of the branching waveguides, and electrodes that give phase change to the light transmitted through the branching waveguides. When the light streams from the branching waveguides converge in phase (with a phase difference of 2 n.pi., n being an integer), the light output is turned on; when the light streams converge opposite to each other in phase (with a phase difference of (2n+1).pi., n being an integer), the light output is turned off. Thus intensity modulation is performed by varying the voltage given to the electrodes by use of an input signal. Where the voltage fed to the electrodes varied as per the input signal, it is necessary to compensate for the operating point drift of the optical modulator caused by temperature fluctuation and other ambient conditions. One way to do this is first to supply the electrodes with a bias voltage such as to keep the operating point where optimum and then to superimpose a signal onto the bias voltage so that the output light is turned on and off while the operating point is being held in its optimum position. Prior art techniques of the above kind for stabilizing the operating point of the optical modulator are described illustratively in Japanese Patent Laid-Open No. 49-42365 and in Japanese Patent Laid-Open No. 3-251815.
Also known is a Mach-Zehnder optical modulator of a symmetrical dual electrode driving type designed to lower the voltage to a signal electrode of the optical modulator and to eliminate chirping in the modulated output light. That is, the branching waveguides have a signal electrode each. These electrodes are fed with driving voltages opposite to each other in phase. This technique is disclosed illustratively in Technical Digest of IOOC' 89, 19D4-2, 1989, "Perfectly Chirpless Low Drive Voltage Ti:LiNbO.sub.3 Mach-Zehnder Modulator with Two Traveling-Wave Electrodes".
U.S. patent application Ser. No. 07/662,412 discloses related techniques of the optical transmitter permitting the selection of the presence of absence of chirping.
The driving signal for optical modulation has a repetitive pulse waveform or an AC waveform, while the bias voltage for operating point control comes from a DC source. Thus the circuit for controlling the operating point is connected to the electrode in a DC setup, whereas the driving circuit is connected to the electrode in an AC setup via a capacitor for DC decoupling. Where the electrode of the optical modulator is built as a traveling-wave type, the electrode is connected to the terminating resistor also in an AC setup. If there exists a capacitor between the driving circuit and the electrode and/or between the terminating resistor and the electrode, the low-frequency component of the driving signal is cut off. This can promote distortion of the signal waveform of the modulated light upon abrupt change in the mark rate. If the frequency characteristic of the capacitor is insufficient, the waveform of the driving signal of as high as several Gb/s is distorted by the capacitor. That in turn distorts the waveform of the modulated light.
Whereas the symmetrical dual electrode type optical modulator may lower the driving voltage, its application to the optical transmitter with a stabilized operating point is yet to be implemented extensively.